that discourage or prevent the growth of microorganisms that cause spoilage. The major inﬂuences on the growth and reproduction of microorganisms in grain include: moisture, temperature, oxygen supply, pH, condition of the grain, storage time, initial infestation and the amount of foreign matter present. Aerating stored grain, whether it is contained in bags, boxes, concrete or steel silos, or ﬂat storage buildings, helps to maintain its quality. Before the principles of aeration were known or aeration equipment developed, the only method available to storage operators was to turn the grain in the storage system, thus providing some contact with fresh, cooling air. This method required additional storage space, machine and manpower for moving the grain, causing wear and tear on machinery and creating stress cracks and handling damage in the grain itself. This increased the percentage of fne material in the total grain mass. Grain storage aeration systems have four basic functions. These systems: Remove storage odours; Aeration systems will remove undesirable odours from mouldy, sour, or fermented grain. Protect against mould growth. High grain temperatures and moist grain conditions encourage the growth of mould and fungi. Lower grain temperatures, below 21°C, discourage this growth. Fungi growth rates decrease to a minimum from 2° to 5°. Hinder insect activity. Insects multiply rapidly in grain when the grain temperatures exceed 20°C. In general, insect reproduction is low when grain temperatures are below 15° to 16° C. Low temperatures impede insect movement, feeding and reproduction. However, lowering grain temperatures just 10° C (from maximum levels) can greatly reduce the movement and growth of some types of grain pests (others are affected differently), even if ambient atmospheric conditions do not allow the lower grain temperatures to the levels suggested above. If it is possible to lower grain temperatures to 4° - 5° C, the resident insect population in the grain mass may starve and die. If climatic conditions at the storage site offer these kinds of temperature possibilities, it may be possible to avoid costly grain fumigation to deter insect infestations. Aeration is not, however, a total substitute for fumigation or good management practices. Resist moisture migration and accumulation. Grain placed into storage at harvest time is usually quite warm. Grain acts as an insulator, meaning that heat dissipates slowly from the interior of the grain mass. In areas with four seasons that include cool fall and winter periods, grain within one to two feet (3000 to 600mm) of the outside wall cools, while grain in the centre of the silo remains at higher harvest temperatures.
Cool air in the grain mass near the silo walls moves downward, forcing warm air upward through the centre of the grain mass. Simple psychometrics explains that this warm rising air has more capacity to absorb moisture than cool air. It therefore absorbs moisture from the grain. When warm, moist air rising in the grain mass makes contact with the cool grain surface at the top of the silo and condensation may (depending on temperatures) occur in the same way moisture condenses on the exterior of a glass of ice water. This sometimes causes a crust to form on the top surface of the grain and, if severe enough, can create a small amount of sprouting.